The human interleukin-2 receptor is being studied to understand critical components of the T cell immune response in normal and neoplastic cells. Following T-cell activation, IL-2 and IL-2 receptors are induced; the magnitude and duration of the T-cell immune response is controlled by the amount of IL-2 produced, the levels of receptors expressed, and the time course of these events. Three chains of the IL-2 receptor exist, IL-2Ra, IL-2Rb, and gc, with IL-2Ra and IL-2Rb being significantly regulated at the level of transcription. The group has focused primarily on the types of signals induced by IL-2, particularly the activation of STAT proteins, and the mechanism of regulating IL-2Ra gene expression in response to mitogen and IL-2. Considerable progress has been made in analyzing the STAT proteins (signal transducers and activators of transcription) induced by IL-2. IL-2 can activate both Stat5a and Stat5b (two closely related proteins with >90% amino acid identity) in fresh human peripheral blood lymphocytes (PBL) and additionally activates Stat3 in PBL preactivated with phytohemagglutinin. The analysis of Stat5a and Stat5b knockout mice was continued. Previously, it was shown that like Stat5a knockout mice, Stat5b knockout mice also exhibit a substantial defect in the proliferative and cytolytic activities of natural killer cells. In the past year, it was demonstrated that both Stat5a and Stat5b are required for antigen-induced eosinophil and T-cell recruitment into tissue. Consistent with the presence of an IL-2 response element in the 5' regulatory region of the IL-2Ra gene, it was previously shown that like Stat5a knockout mice, Stat5b knockout mice are defective in IL-2-induced IL-2Ra expression. Transgenic approaches are being used to further investigate distinctive vs. overlapping roles of Stat5a and Stat5b. In collaborative studies, the group reported the effect of HMG-I(Y) proteins on imparting architectural specificity to a positioned nucleosome on the IL-2 receptor alpha chain promoter. Moreover, an additional regulatory element in the IL-2Ra gene has now been characterized. Furthermore, it was reported that Stat5a and Stat5b proteins purified from a baculovirus expression system were used in a binding site-selection analysis. This has revealed that both Stat5a and Stat5b dimers bind to similar TTCN3GAA motifs, whereas Stat5a tetramers exhibit binding to a larger repertoire of sites than was expected. Specifically, it was demonstrated that in addition to canonical GAS motifs, divergent "half-GAS" motifs or even less similar sequences importantly contributed to binding. The larger repertoire of sites also likely serves as a basis for achieving greater specificity of binding of particular STAT proteins to particular promoters. The lab has also worked to set up systems to define the role of serine phosphorylation of substrates in response to IL-2, and to evaluate the functional significance of serine phosphorylation of stat5 proteins. Together, these studies substantially enhance our understanding of the basis for IL-2R expression as well as IL-2-dependent gene regulation.